Two-color electrostatic printing apparatus



July 24, 1962 F. A. scHwl-:RTZ

TWO-COLOR ELECTROSTATIC PRINTING APPARATUS Filed June 6, 1957 July 24, 1962 F. A. scHwERTz TWO-COLOR ELECTROSTATIC PRINTING APPARATUS Filed June 6, 195'? 2 Sheets-Sheet 2 r M m w m ii-1:. E-

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iiriited States Patent Gi 3,045,644 TWO-COLR ELECTROSTATIC PRINTING APPARATUS Frederick A. Schwartz, Pittsford, N.Y., assignor t Xerox Corporation, a corporation of New York Filed June 6, 1957, Ser. No. 664,136 Claims. (Cl. 118-637) The present invention relates .generally to electrostatic printing techniques and more particularly to apparatus for recording alphanumerical and blockinformation electrostatically and for producing prints thereof in two colors.

1n the copending application Yentitled Electrostatic Alphanurnerical Printer with Image Transfer Mechanism, Serial No. 664.138, filed June 6, 1957, now U.S. Patent No. 3,023,731, there is disclosed an arrangement wherein information received from a computer or other data source is recorded electrostatically on a cylinder or endless web having a dielectric surface. The information, which may be binary in form, is fed to a character-forming station constituted by a series of heads, each head including an array of pins which may be selectively actuated by pulses to build up a dot pattern of charges representative of the input information.

The recording cylinder is rotated relative to the character station whereby successive lines of characters are formed thereon. A charging station is disposed in advance of the character station to pre-stress the dielectric layer on the cylinder to a uniform potential, thereby erasing previously formed images. After recording, the cylinder layer passes through a developer section to form powder images, and the powder images are then transferred by electrostatic attraction to a paper strip on which the powder is fused to provide a permanent print.

The apparatus disclosed in said copending application makes use of the so-called Tesip-rinting technique (gransfer Electro tatic image) in which, as distinguished from the electophotographic process in Xerography, physical symbols or characters rather lthan light patterns are recorded as electrostatic images. In this apparatus the powder applied to the image is of a single color and the resultant print is of the same color.

When the printer is used to record accounting information, it is frequently desirable that the data be printed in two colors, say red and black. Color distinctions are valuable not only in accounting procedures but in many other computing systems. It is also useful to have the recorded information entered in a suitable form, such as a bookkeeping page or the like.

Accordingly, it is the main object of this invention to provide an electrostatic printer capable of printing in two colors.

More particularly it is an object of the invention to provide a two-color electrostatic apparatus wherein data is recorded by the Tesi technique on an insulating medium to form latent electrostatic images having a charge polarity depending on the desired color. In accordance with the invention, negatively charged toner of one color is applied to the positively charged images and positively charged toner of another color is applied to the negatively charged images to produce distinct powder images in two colors on the recording medium. These two-color powder images are then transferred to a paper strip and fused thereon.

Also an object of the invention is to provide a twocolo-r printing apparatus wherein business forms may be applied optically by the Xerogra-phic technique to a photoconductive recording surface and `data may be entered in the Xerographic images of the form by the Tesi technique.

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Brieily stated, in two-color electrostatic apparatus for recording and printing alphanumerical information in accordance with the invention, the information received from a computer or other data source is recorded electroi statically on an endless medium having an insulating sur*- face of high resistivity for sustaining electrostatic charges. A character-forming station is supported in spaced relation to the insulating surface of the recording medium to define an air gap therebetween. The character station includes character-forming elements adapted to produce latent electrostatic images on said medium when voltages are applied thereto to effect a field discharge in the air gap. The character-forming elements are constituted by separate recording heads each having an array of pins which may be selectively actuated by pulses to build up the character by 4a pattern of charge dots. Characters in one color are formed by charges of a given polarity and characters of another color are formed by charges of the reverse polarity.

The medium is moved relative to the character station to produce successive lines of characters thereon. An alternating-current charging station is disposed in advance of the character station to erase both positive and negative latent images previously formed, there neutralizing the medium in preparation for the next cycle of operation. The medium after recording passes through a developer section to form two powder images of different color and opposing polarity. TheA` powder images are thereafter sprayed with a corona discharge current to impart a charge of one polarity to all ofthe powder images. The powder images are then transferred by electrostatic attraction to a paper strip and are fused thereon to provide a permanent print. n For a better understanding of the invention, as well as y further objects and features thereof, reference is made to the following detailed description to be read in connection with the accompanying drawing, wherein like components in the several views are identified by like reference numerals.

In the drawings:

FIG. 1 is a schematic diagram, in perspective, showing one preferred embodiment of a two-color printer in accordance with the invention.

FIG. 2 is a schematic diagram, in perspective, showing a second preferred embodiment of a two-color printer in accordance with the invention.

FIG. 3 shows a third preferred embodiment of the invention.

Referring now to FIG. 1, the two'color electrostatic printer according to the invention comprises a rotary metal cylinder 10, which is coated with a layer 11 of dielectric material having a high resistivity for holding an electrostatic image. The ycylinder moves in a counterclockwise direction at `a uniform velocity.

A character printing station 12 is provided constituted by a bank of stationary recording heads 13 having electrode points or pins 14 which can be selectively pulsed to provide a matrix type of presentation simulating an alphanumerical character. IOne head is provided `at each column position. As shown in the drawing, the characters are each formed by `a pattern of dots.

Each recording head 13 consists of a set of five flat faced pins 14 in a row maintained `at a fixed distance from the surface of the cylinder layer 11, the pins of the several heads being aligned and running across the cylinder. The individual characters are built up by lseven successive choices of the tive pins as the cylinder moves forward. An electronic pulsing circuit is provided for each pin. Thus as the cylinder rotates, the pins in the heads are selectively activated by input information in seven successive steps to construct latent images of the appropriate characters on the cylinder surface.

accese-e A magnetic matrix and `decoding unit 15 may be used to buffer the pulsing circuits to the pins and the information source. A description of a unit -suitable for this purpose as well as an electronic pulsing circuit for the pins may be found in the article Burroughs Electrographic Printing Technique by Herman Epstein, March 1955 Proceedings of the Western Joint Computer Conference (AIEE-IRE).

The Tesi technique may be employed to produce either negative or positive electrostatic images. This makes possible two-color printing, the characters to appear in one color, say red, being formed as negative charges and the characters to appear in the second color, say black, being formed `as positive charges. In practice this is accomplished in the recording heads by pulsing the pin electrodes of those heads which record in red with negative voltages relative to the metal cylinder, and pulsing the pin electrodes of the remaining heads which are to record in black with positive pulses relative to the cylinder. Thus separate areas carrying negatively and positively charged latent images are formed on the dielectric surfaces of the cylinder.

The latent images are developed by magnetic brush elements 16 and 17, one for each color, the brushes being positioned in tandem relation relative to the cylinder at a point beyond the character-forming station. The negative areas are developed by a toner which is triboelectrically positive and the positive areas are developed by a triboelectrically negative toner. The developer powders or toners utilized in the magnetic brushes are composed of nely divided magnetic particles and still finer toner particles consisting of thermoplastic pigmented materials which are charged triboelectrically by Contact with the magnetic material.

The core particles may be of ferrite material or of any known magnetic substance such as iron, the toner may be constituted by a colored insulating material selected for its triboele-ctric characteristics. The magnetic cores of the two toners of the two brushes may be identical and the triboelectric effect may be controlled by proper selection of the coating material.

When the ferromagnetic powder is picked up by a permanent magnet, the powder particles arrange themselves along the magnetic lines of force in long chains to simulate the bers of a brush. When this simulated brush passes over a charged area on the recording medium, the electrostatic attraction between the charged toner particles and the oppositely charged image areas is greater than the attraction between the toner and the brush magnet. Consequently the toner is stripped from the brush and adheres strongly to the charged insulating surface on the cylinder, thereby forming the powder image. The developer mix is thus handled and brought into position by magnetic attraction, whereas the toner image is built up by electrostatic forces.

`Magnetic brush 16 carries negatively charged toner of black powder which adheres to the positively charged areas on the cylinder and is repelled by the negatively charged aras, while magnetic brush 17 carries positively charged toner of red powder which adheres only to the negatively charged areas. Obviously, the invention is not limited to the red and black, and any contrasting choice of colors may be used.

The permanent magnet may be held at ground potential or biased, as desired, to achieve optimum results'. Alternatively, it is possible to use a single magnetic brush which itriboelectrically charges the two toners to opposite polarity.

The biasing voltage cooperates with the electric field of the electrostatic image to overcome the electrostatic attraction of the magnetic brush so that the desired toner particles are released from the permanent magnet and deposited on the electrostatic image to develop it into a powder image. Both the electrostatic latent image and the magnetic brush compete electrostatically for the nonmagnetic toner particles. When the brush bias is of opposing polarity from the electrostatic image, a direct powder image is developed, but if the bias is of the same polarity and of sufficient magnitude to overcome the electrostatic and magnetic forces, a reverse image is developed.

Before the powder images can `be transferred electrostatically, the net charge on all the toner particles must be of the same sign. This is accomplished by means of a corona spray station 18 which acts to spray the images with a corona discharge current imparting a like charge to the toner images.

After leaving the corona spray station 1S, the powder images of the same sign formed on the surface of the cylinder are transferred to a paper strip 19 which is fed from a suitable supply roll 20, the strip travelling over a transfer roller 21 and being rewound on a take-up roller 22.

The transfer roller 21 is supported transversely with respect to the lower end of the cylinder 10 and acts to carry the paper strip into momentary contact with the dielectric surface, the cylinder and strip moving in like directions at the same linear speed. The only point of contact between the paper strip 19 and the cylinder surface is at the position of the transfer roller.

The transfer Vroller 21, which may be made of conductive rubber material, is connected to a voltage source 23 which functions to impress a charge across the region of the paper strip 19 engaging the cylinder. The magnitude of the charge applies to the paper strip and the polarity thereof is such as to overcome the adhesion 0f the powder particles to the image area on the cylinder and to establish a greater tendency of the particles to adhere to the paper, thereby effecting an offset of the powder image to the paper strip. The charge applied to the paper strip 'by the transfer roller is of opposite polarity to the charge on the developer powder, the powder therefore being attracted to and retained by the paper strip.

The powder image transferred to the paper strip is fused thereon 'by heating elements 24 of any suitable construction disposed in operative relation to the strip between the transfer roller 21 and the take-up roller 22.

To remove residual powder from the cylinder after the transfer operation is completed and 'before it again enters the character-forming station 12 for the next recording cycle, a rotary fur brush 25 is provided which engages the surface of the cylinder. The brush acts to remove any powder adhering to the insulating surface which has not been previously transferred to the 'Paper strip, thus preparing the surface for a fresh supply of powder. rThe cleaning means may consist of any suitable wiping or suction device and is preferably of the type `disclosed in the patent to Turner et al. No. 2,751,616, issued June 26, 1956.

After the surface has been physically cleaned of residual toner a corona charging station 26 in advance of the character-forming station 12 acts Iby means of corona wires energized 'by alternating current to remove both positively and negatively charged images in preparation for the next cycle. Thus the apparatus in FIG. 1 functions automatically and at high speed to record electrostatically and to print on a transfer strip information in two colors.

Referring now to FIG. 2, there is shown a two-color printer which makes use both of xerographic and Tesi recording techniques to print data on prepared forms selected from a store thereof. The forms may, for eX- ample, be business forms from which a random selection is made. The store ymay consist of a group of IBM cards on each of which is an `opaque image of the business form or on which a microlm of vthe form is mounted. Such lm record cards are illustrated in the Longan Patents Nos. 2,512,106 and 2,511,859. Alternatively, the store may consist of an endless loop of lm which may be searched photoelectrically.

The printer in FIG. 2 is identical with that in FIG. l save for the fact that the cylinder 16 is coated with a photoconductive insulating surface 27, rather than a purely dielectric surface. The surface may ybe fabricated of a selenium layer which accepts both positive and negative charges equally well.

As the cylinder rotates in the counterclockwise direction, an image of the form to be printed is optically scanned at a given rate and is projected through a suitable slit, represented schematically by dotted line 2S, onto the selenium surface at a position intermediate the corona charging station Z6 and the character-forming station 12.

The optical image projected on the photoconductive surface of the cylinder discharges selected portions of the n charged surface to produce thereon an electrostatic image corresponding substantially to the projected radiation image. 'Ibis procedure is in accordance with well known Xerographic techniques and the exposure leaves the latent electrostatic image of the rbusiness: form as say a positively charged pattern, whereas the background areas are essentially reduced .to lground potential.

As the drum continues to rotate variable alphanumerical infomation is introduced into the background areas at the character-forming station 12 by means of the Tesi technique, the characters being of positive and negative charge in accordance with color requirements. The form-ation of the images at the optical projection and the characterforming stations must of course tbe carefully synchronized to prevent image overlap.

The combined latent images then travel under the magnetic brushes 16 and 17 where colored toner is applied in keeping with the image area polarities. The resultant powder images are thereafter transferred to the paper strip 19 and Xed permanently thereon in the manner previously described in connection with FIG. l.

In the printer disclosed in FIG. 2, because the recording surface is cylindrical, slit projection is necessary to form the optical image thereon. In order to provide an extended flat surface permitting frame projection of the form, the printer illustrated in FIG. 3 includes a recording surfacein the form of a flexible endless belt 3@ of conductive material having a photoconductive layer 31 coated thereon. Therecording belt is supported for movement on three rollers 32, 33 and 34 triangularly ar-y ranged.

Recording takes place on the web in the broad vertical expanse `between rollers 32 and 33. In certain situations it may be desirable to insert certain portions of the variable information in parallel or block form by optical projection instead of serially, character by character. In this case it is advantageous to have at least two projectors 35 and 36 for projecting frame images on the web surface. Projector 35 may be used to insert a fixed business form and projector 36 to insert variable information such as addresses, say with the aid of IBM cards. By reason of the flat flexible web, several projection stations may be employed to record by the Xerographic process on the web.

This arrangement considerably simplifies the handling of input information. It has been established, for example, that a selenium coated web may be exposed effectively in as little as 40 microseconds with the aid of a bright light source. During a time interval of this brief duration, the motion of the web is negligible and no perceptible image blurring is encountered.

The characters are impressed by the Tesi technique at the character-forming station containing a row of recording heads, one for each column. Since a flat web is employed, in lieu of recording heads a character drum may be employed containing character rings insulated from each other, in the manner disclosed in the aboveidentified copending application. Positively and negatively charged images may be introduced and these images developed in two colors and transferred to a paper strip in the manner described in connection with FIG. 1.

While there has been shown what are considered to be preferred embodiments of the invention, it will be appreciated that many changes and modifications may be made therein without departing from the essential spirit of the invention. It is intended in the appended claims to cover all such changes and modifications as fall within the true spirit of the invention.

fWhat is claimed is:

1. A two-color electrostatic apparatus for recording and printing information and comprising a movable endless recording medium having an insulating surface for sustaining electrostatic charges, means to produce composite negatively and positively charged latent electrostatic images on said medium, the charges of said latent electrostatic images having a polarity depending on the desired color, means operatively disposed with respect to said medium beyond said station and including powdered toners of contrasting color and opposing polarities to form powder images of said latent images with like charges being developed in the same color, means to impart a charge of one polarity to said powder images, and means operatively coupled to said medium to transfer said powder images onto a web by electrostatic attraction.

2. A- two-color electrostatic apparatus for recording and printing information and comprising a movable endless recording medium having an insulating surface for sustaining electrostatic charges of negative and positive polarity, a character-forming station supported in spaced relation to said insulating surface to define an air'gap therebetween and including character-forming elements adapted to produce composite latent electrostatic images of negative and positive charges on said medium when voltages are applied thereto effecting a field discharge in said air gap, said latent images having a polarity depending on the desired color, means to apply voltages of one polarity to selected elements of said character forming station and voltages of opposite polarity to selected elements of the remaining elements of said character forming station, means operatively disposed with respect to said medium beyond said station and including powdered toners of contrasting color and opposing polarities to form powder images of said latent images with like charges being developed in the same color, means to impart a charge of one polarity to said powder images, and means operatively coupled to said medium to transfer said powder images onto a web by electrostatic attraction.

3. A two-color electrostatic apparatus for recording and printing information and comprising a movable endless recording medium having an insulating surface of high resistivity for sustaining electrostatic charges, a character-forming station supported in spaced relation to said insulating surface to define an air gap therebetween and including a bank of character-forming elements to produce composite latent electrostatic images of negative and positive charges on said medium when voltages of different polarity are applied thereto effecting a field discharge in said air gap, means to apply voltages of one polarity to selected elements in said bank and voltages of opposite polarity to selected elements of the remaining elements in said bank, charging means operatively disposed with respect to said medium in advance of said station to neutralize the charge thereon, first and second magnetic brush developer means operatively disposed with respect to said medium beyond said station to form powder images of said latent images with like charges being developed in the same color, said brushes supplying toners of opposing polarities and contrasting color, means to impart a charge of one polarity to said powder images, and means operatively coupled to said medium to transfer said powder images onto a paper strip by electrostatic attraction.

4. A two-color electrostatic apparatus for recording and printing inform-ation and comprising an endless recording medium having an insulating surface of high resistivity for sustaining electrostatic charges, a character-forming station supported in spaced relation to said insulating ati/tassa surface. to define an air gap therebetween and including a bank of character-forming elements adapted to produce composite latent electrostatic images of negative and positive charges on said medium when voltages are applied thereto effecting a field discharge in said air gap, means to apply voltages of one polarity to selected elements in said bank and voltages of opposite polarity to selected elements of the remaining elements, means to move said medium relative to said character-forming station to form successive lines of latent images thereon, charging means operatively disposed with respect to said medium in advance of said station to neutralize the charge thereon and thereby erase previous charges, first and second magnetic brush developer means operatively disposed with respect to said medium beyond said station to form powder images of said latent images with like charges being developed in the same color, said brushes supplying toners of opposing polarities and contrasting color, means to supply a charge of one polarity to said powder images, and means operatively coupled to said medium to transfer said powder images onto a paper strip by electrostatic attraction.

5. A two-color electrostatic apparatus for recording and printing alphanumerical information at high speed, said apparatus comprising an endless recording medium` having an insulating surface of high resistivity for sustaining electrostatic charges, a character-forming station supporti ed in spaced relation to said insulating surface to define an air gap therebetween and including a bank of characterforming elements adapted to produce latent electrostatic images on said medium when voltages are applied thereto effecting a eld discharge in said air gap, means to apply voltages of one polarity to selected elements in said bank and voltages of opposite polarity to selected elements of the remaining elements, means to move said medium relative to said characterforming station to form successive lines, alternating-current corona charging means operatively disposed with respect to said medium in advance of said station to neutralize the charge on said medium and thereby erase previous charges, first and second magnetic brush developer means operatively disposed in tandem with respect to said medium beyond said station and including toners of opposing charge to form powder images of said latent images; means to supply a charge of one polarity to said powder images, means operatively coupled to said mediumto transfer said powder images by electrostatic attraction, said lastdlamed means including a paper strip movable in synchronism with said web, a transfer roller for effecting engagement between said strip and said web, and means to apply a potential to said transfer roller to attract said powder images to said strip.

6. High-speed apparatus for recording and printing alphanumerical information in two colors comprising a rotary metal cylinder having a dielectric layer coated thereon for sustaining electrostatic charges, a character recording station including a bank of stationary character heads extending across said cylinder, each head including a row of point electrodes disposed in spaced relation to said dielectric layer to define an air gap, means to rotate said cylinder relative to said recording station at a constant velocity, means selectively to pulse said point electrodes durin" movement of said cylinder in accordance with the value of applied information to be recorded to form an electrostatic point charge pattern on said layer simulating a character representative of said value, some of said heads being pulsed in voltages of one polarity, the others being pulsed in the opposing polarity in accordance with color requirements, developer means including toners of opposing polarities to form powder images of said charge patterns, means to impart a charge of one polarity to said powder images, means to transfer said powder images to a paper strip by electrostatic attraction, and pre-charging means to return said layer on said cylinder to a uniform potential after said transfer operation.

7. Highspeed apparatus for recording and printing alphanumerical information in two colors comprising a rotary metal cylinder having a dielectric layer coated thereon for sustaining electrostatic charges of opposing polarities, a character' recording station including a bank of stationary character heads extending across said cylinder, each head including a row of point electrodes disposed in spaced relation to said dielectric layer to define an air gap, means to rotate said cylinder relative to said recording station at a constant velocity, decoder means selectively to pulse said point electrodes during movement of said cylinder in accordance with the value of applied information to be recorded to form an electrostatic point charge pattern on said layer simulating a character representative of said value, some of said head being pulsed in voltages of one polaritt the others being pulsed in the opposing polarity, developer means including magnetic brushes carrying toners of opposing polarities, to fori powder images of said charge patterns, corona spray means to impart a single charge to said powder images, means including a biased transfer roller to transfer said powder images to a paper strip by electrostatic attraction, and pre-charging means to return said layer to a uniform potential after said transfer operation.

8. High-speed apparatus for recording and printing information in two colors comprising a rotary metal cylinder having a photoconductive dielectric layer coated thereon for sustaining electrostatic charges, slit projection means optically to cast a form on said layer to produce a Xerographic image thereon, a character recording station including a bank of stationary character heads extending across said cylinder, each head including a row of point electrodes disposed in spaced relation to said dielectric layer to define an air gap, means to rotate said cylinder relative to said recording station, means selectively to pulse said point electrodes during movement of said cylinder in accordance with the value of applied information to be recorded to form an electrostatic point charge pattern on said layer simulating a character representative of said value, some of said heads being pulsed in one polarity and others in the opposing polarity in accordance with color requirements, developer means including first and second magnetic brushes carrying powder toners of opposing polarities and contrasting color operatively disposed relative to said cylinder beyond said station to form powder images of said charge patterns with like polarity charges being developed in the same color, means to impart a common charge to said powder images, and means including a charged transfer roller to transfer said powder images to a paper strip by electrostatic attraction.

9. High-speed apparatus for recording and printing information in two colors comprising an endless metal belt having a photoconductive dielectric layer coated thereon for sustaining electrostatic charges, frame projection means optically to cast a form on said layer to produce a Xerographic image thereon, a character recording station including a bank of stationary character heads extending across said cylinder, each head including a row of point electrodes disposed in spaced relation to said dielectric lfyer to de'rlne an air gap, means to move said belt iele.ive to said recording station and said projection means, means selectively to pulse said point electrodes during movement of said belt in accordance with the value of applied information to be recorded to form an electrostatic point charge pattern on said layer simulating a character representative of said value, said character being entered on said form, some of said heads being pulsed in one polarity and others in the opposing polarity, developer means including rst and second magnetic brushes carrying powdered toners of opposing polarities and contrasting color operatively disposed relative to said cylinder beyond said station to form powder images of said charge patterns with like charges being developed in the same color, means to impart a common charge to said powder images, and means including a charged trans- References Cited in the le of this patent UNITED STATES PATENTS 2,221,776 Carlson Nov. 19, 1940 2,297,691 Carlson Oct. 6, 1942 2,357,809 Carlson Sept. 12, 1944 2,703,280 Buttereld et al. Mar. 11, 1955 10 Buhler Dec. 13, 1955 Carlson Sept. 4, 1956 McNaney Jan. 15, 1957 Lowrie Aug. 20, 1957 Fitch Sept. 24, 1957 Schroeder Feb. l1, 1958 Wilson Aug. 5, 1958 Bolton Oct. 21, 1958 Lehmann Mar. 24, 1959 Giaimo June 16, 1959 Sugarman Nov. 24, 1959 Schwertz Ian. '5, 19160 

